1. Field of the Invention
The present invention relates to a photo lithographic mask, also called a reticle, used in a photolithography process for a semiconductor wafer. More particularly, the present invention is directed to a design of a reticle used for a photographic process for the manufacture of semiconductor chips.
2. Description of the Related Art
Manufacturing semiconductor chips involves a process called photolithography. In a typical photolithography process, a thin layer of a photosensitive material or photo resist is deposited over a semiconductor wafer. During the photolithography process, illumination such as ultra-violet light is illuminated through a lens system and a photo lithographic mask or reticle to the semiconductor wafer. The reticle has a particular device pattern and the pattern is exposed over a portion of the wafer by the illumination to create exposed and unexposed regions on the wafer. Then these exposed or unexposed regions are washed away to define circuit elements on the wafer. This photolithography process is repeated many times to define many circuit elements on the wafer. At the end of the photolithography process, the wafer having the exposed device pattern is cut into semiconductor chips.
Typically, a reticle is made from a transparent plate and has a device exposure region and an opaque chrome region. The plate is often made of glass, quartz, or the like, and the opaque chrome region typically includes a layer of chrome. The device exposure region generally has a square or rectangular shape and is positioned in the center of the reticle. The device exposure region includes transparent portions and opaque portions defining a device pattern. The transparent portions in the device exposure region allow illumination from the light source to travel though them and reach the wafer. On the other hand, the opaque portions in the device region block the light and the light does not reach the wafer, and this exposing the device pattern on the wafer.
FIG. 1 illustrates a conventional reticle. FIG. 2 is a magnified view of the circled portion II of the reticle in FIG. 1. The reticle 10 has a square device region 12 surrounded by an opaque chrome region 14. For the sake of simplicity, a device pattern 13 in the device region is not illustrated in detail in FIGS. 1 and 2. As illustrated in FIG. 2, there is a kerf region 16 at the periphery of the device region 12 between the device region 12 and the opaque chrome region 14. The kerf region 16 contains important information regarding the photolithography process of the wafer. (not illustrated in the drawings) The kerf region typically includes test structures to verify the performance of a photolithography process. For example, the kerf region may include alignment marks to check the accuracy of the reticle alignment and registration marks to measure the resolution of the device pattern during the photolithography process. After the photolithography process, the wafer is diced into semiconductor chips through the kerf region. FIG. 3 shows a portion of a wafer after the photolithography process using the kerf shown in FIGS. 1 and 2. FIG. 3A illustrates a magnified portion of the wafer. Four device regions 12 separated by the four contiguous kerf regions 16 are shown in FIG.
The recent growth in the demand for more powerful, faster computer chips requires smaller. and more devices in a semiconductor chip. As a result, semiconductor manufacturers are driven to produce more dense semiconductor chips by, for example, improving resolution capability of photolithography machines.
One possible method to accentuate the resolution capability of a photolithography machine is a xe2x80x9cdouble exposure method.xe2x80x9d In the double exposure method, a wafer is exposed at a normal dose of light through a reticle. Then the wafer is slightly shifted with respect to the reticle, and subsequently exposed again at a nominal dose of the light through the reticle. A device structure is created as a result of a sum of the two exposures. This double exposure method can enhance the resolution capability of the photolithography machines, and facilitate more dense semiconductor chips.
This double exposure method, however, presents another challenge to the photolithography process. Though the device exposure region is exposed two times in the double exposure method, the kerf region, containing the test structures, cannot be exposed multiple times. In another words, one area of the exposure region in a wafer needs to be doubly exposed; on the other hand, another area of the exposure region needs to be exposed only a single time.
A device called a reticle blind can be used to block the kerf region of the reticle to prevent them to be exposed for the second time. In the conventional reticle, as shown in FIGS. 1 and 2, the kerf region 16 is located adjacent to the device region 12. Current lithography machines, however, do not have a reticle blind to accurately block the kerf region 16 and the opaque chrome region 14, and expose only the device region 12.
Therefore, there is a need for a reticle that can accentuate the resolution of a photolithography machine and yet meets the challenge associated with the double exposure method within mechanical constraints of the current photolithography machines.
The object of the invention is to facilitate the double exposure method of a photolithography process by addressing the above-identified problems associated with the double exposure method.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purpose of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, the invention includes a reticle for making a semiconductor device in a photolithography process. The reticle has a device exposure region having sides and a device pattern within an, area defined by the sides, an opaque chrome region disposed adjacent to the device region, and a kerf region surrounded by the opaque chrome region. The kerf region is offset from the sides of the device exposure region by the opaque chrome region.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.